This invention relates generally to photodrafting equipment, that is to say, equipment arranged to project a beam of light to illuminate a spot on a photosensitive surface and to move the spot of light relatively to the surface to expose a line thereon. More particularly, the invention relates to projection apparatus for photodrafting equipment.
Photodrafting equipment is commonly used to expose photographic sheets used in turn to produce masks for the production of printed circuit boards and the like. Projection apparatus mounted on a carriage movable in X- and Y- co-ordinate directions under the control of a computer or other control means is arranged to project a beam of light substantially orthogonally against a photographic sheet disposed in a plane parallel with the X- and Y-coordinate directions. By this means, a line can be exposed on the sheet by movement of the carriage during continuous projection of the beam, or discrete "pads" can be exposed in selected positions on the sheet by moving the carriage to the corresponding coordinate positions successively, the light beam being projected only when the carriage is located at one of those positions.
It is to be noted that, when exposing a line the exposure of any given section of the line depends upon the speed with which the spot of light formed by the beam is moved along that section of the line. To compensate for variations in exposure during acceleration and deceleration of the carriage at the start and finish of a line, the intensity of the beam can be varied in accordance with the speed of movement of the carriage.
Conveniently, the beam of light projected when exposing lines is circular in cross-section with a substantially uniform distribution of intensity across the area of the section. This, however, presents the problem that the amount of light received at the extreme ends of the line is small compared with the amount received by other parts of the line, so that the ends of the line are liable to be underexposed causing fuzziness and indistinctness.